Matter May Get Reprieve from Milky Way's Black Hole

WASHINGTON (January 11, 2006) -- A journey into a black hole is a one-way
trip to oblivion: Matter and energy fall in, but they can’t come back
out. But astronomers say that some of the material that nears the black hole
at the center of the Milky Way galaxy appears to get a last-second reprieve:
It blasts out into space just before it would disappear forever.

The black
hole, known as Sagittarius A* (Sgr A*), is about 3.6 million times as massive
as the Sun. Gas and dust that are spiraling into the black hole form a superhot
disk around it, with temperatures reaching millions of degrees.

A
team of astronomers headed by Farhad Yusef-Zadeh of Northwestern University
watched the center of the Milky Way with six telescopes on the ground and three
more in space. The list included ground-based optical and radio telescopes
plus Hubble Space Telescope, the European XMM-Newton X-Ray Observatory, and
a gamma-ray satellite called Integral. The observations were conducted in March
and September of 2004, with the results reported this month at the American
Astronomical Society conference.

Observations
with these instruments revealed that Sgr A* flares up every half hour. The
flares, which typically last a few minutes, produce every form of energy, from
radio waves through X-rays. The flares appear to be a “way of life” near
the black hole, Yusef-Zadeh said, occurring about a third of the time. Such
flares have not been detected around the supermassive black holes at the centers
of other galaxies, he added.

Additional
support for the flares from the accretion disk around Sgr A* was reported by
astronomers from UCLA and Caltech, who observed the Milky Way’s core
with one of the giant Keck telescopes in Hawaii. Their observations revealed
brief bursts of red light coming from around Sgr A*. The light probably is
emitted by electrons -- the negatively charged particles in atoms -- that are
spiraling in a magnetic field around the black hole.

The
astronomers snapped some of the sharpest images of the center of the galaxy
ever obtained from the ground by using a combination of laser light and a small
mirror attached to the Keck II telescope to compensate for distortions caused
by Earth’s
atmosphere.

The
UCLA astronomers, led by Andrea Ghez, are using this technique to track the
motions of stars quite close to the black hole. Calculating the orbits of these
stars helps astronomers refine their estimates of the black hole’s mass.

The
flares seen around Sgr A* may be produced by a powerful magnetic field that
permeates the disk of material around the black hole. The magnetic field lines
get tangled then suddenly “snap,” blasting hot gas back into space.
This is similar to the mechanism that produces solar flares -- big explosions
of particles and energy from the surface of the Sun.